This invention is in the field of thermoplastic polymeric films which may be used for the packaging of products, especially food products such as meat and cheese. More specifically, the invention is in the field of polymeric thermoplastic films that are thermoformable i.e., may be softened through the application of heat, distorted to a desired shape, and cooled.
Polymeric films are widely used in the packaging field for the packaging of products, especially food products. Films used for the packaging of food generally contain multiple layers, in which each layer adds certain desired physical or chemical properties to the completed film. For example, an “oxygen barrier layer” serves to protect the packaged food from spoiling. Oxygen barrier properties are necessary to protect many packaged foods over an extended shelf life. Additionally, an “abuse layer” may serve to protect the packaged product from physical abuse or stresses caused by the normal handling of the product during packaging, shipping, or during commercialization.
Although each of the layers of a polymeric film serve a particular purpose, the film itself must be stable, and the layers must remain together and not delaminate.
One polymer typically used as a barrier layer is ethylene vinyl alcohol copolymers (“EVOH”) which are also known as saponified or hydrolyzed ethylene vinyl acetate copolymers. In addition to its desirable properties as a barrier to oxygen and other gases, EVOH is also an effective barrier to odors, fragrances, solvents and oils. EVOH also has good properties of processability, i.e. in comparison with other polymers it is relatively easy to process into a layer of a multilayer film. EVOH and EVOH copolymers are generally classified according to ethylene content, for example by mole percent. Typically, as the percentage of ethylene increases in relatively low humidity applications, the gas barrier properties decrease, moisture barrier properties improve, and the resin is more processable. At higher levels of humidity, for example at levels that are common for the packaging of meat, of from 90% to 92% humidity, higher ethylene content results in an increase in the moisture barrier properties.
However, there are in general some difficulties with the use of EVOH copolymer in polymeric films. For example, in comparison with other resins EVOH copolymers have poor impact resistance, poor flex crack resistance, and poor drawability.                In order to protect the EVOH layer, which is as previously noted poor in impact and flex crack resistance, additional layers must be added to cover the EVOH layer. One such layer which may be used to protect the EVOH layer is a layer comprising a polyamide. A polyamide is a high molecular weight polymer having amide linkages along the molecular chain structure. Nylon polyamides, which are synthetic polyamides, have favorable physical properties of high strength, stiffness and abrasion resistance.        
It is known in the art of making polymeric films to construct a multilayer film with a barrier layer of EVOH copolymer sandwiched by layers comprising a nylon polyamide. The following patents are of interest in illustrating the prior art polymeric films having a core layer of EVOH between two layers of a nylon.
Commonly owned U.S. Pat. No. 4,284,674, to Sheptak, teaches a polymeric thermal insulation product with EVOH core layer adhered on each side to a nylon layer. The nylon layer is adhered to another layer of a polyolefin, such as an oriented polypropylene.
Commonly owned U.S. Pat. No. 4,355,721, to Knott, teaches at column 5, lines 44 to 52, a multilayer polymeric film for food packaging having a core barrier layer of EVOH sandwiched between two nylon layers. An adhesive layer and HDPE sealant layer are disposed on one side of one of the nylon layers.
Commonly owned U.S. Pat. No. 4,640,852, to Ossian, discloses a multilayer polymeric film with a core layer of EVOH sandwiched between two nylon layers. The film may have additional fourth and fifth layers of an adhesive and a heat sealable polymer, disposed on one side of the nylon layers, and sixth and seventh layers of an adhesive and a LLDPE or propylene-ethylene copolymer disposed on the other nylon layer.
U.S. Pat. No. 4,695,491, to Kondo, teaches a heat shrinkable composite packaging film. The film has a core layer of EVOH which is surrounded on one side by a first layer of a polymer having a low permeability to hot water, and on the opposing side an outermost layer of antiblocking polymer. The outermost layer may be a nylon.
U.S. Pat. No. 4,729,926, to Koteles, discloses at column 4 a polymeric film having the following structure:tie/nylon/EVOH/nylon/tie/LDPE/primer/outer layerThe outer layer may be any of various polymeric materials that are suitable barrier materials.
U.S. Pat. No. 4,735,855, to Wofford, teaches a seven-layer thermoformable polymeric laminate having the following layer structure:abuse layer/tie/nylon/EVOH/nylon/tie/sealantThe sealant layer may be any of various sealants, including an ionomer. The abuse layer also serves as a moisture barrier layer.
U.S. Pat. No. 4,746,562, to Fant, disclose s a seven-layer polymeric film having the following layer structure:LLDPE/tie/nylon/EVOH/nylon/tie/LLDPEEach of the outer LLDPE layers also comprises an antiblocking agent.
U.S. Pat. No. 4,755,419, to Shah, discloses an oxygen barrier oriented seven layer heat-shrinkable film of the following layer structure:blend/tie/nylon/EVOH/nylon/tie/blendThe blend used for the outer layers may be a blend of LLDPE, LMDPE and EVA. Alternatively, the outer layers may comprise a blend of ethylene propylene copolymer or polypropylene.
U.S. Pat. No. 4,788,105, to Mueller, teaches an oxygen barrier film which is adhered through use of an adhesive layer to a second film comprising a nylon. The film may also comprise an LLDPE outer layer.
U.S. Pat. No. 4,816,304, to Nohara, teaches a multi-layer gas barrier vessel with a core layer of EVOH sandwiched between two nylon layers, an outer layer of polyester, and an inner layer of polyester.
Commonly owned U.S. Pat. No. 4,818,592, to Ossian, teaches a core layer of EVOH sandwiched between two nylon layers. The film may also include a fourth layer of an adhesive and a fifth layer of a heat sealable polymer. In an alternative embodiment the film may also include a sixth layer of an adhesive and a seventh layer of a heat sealable polymer disposed on the fifth layer.
U.S. Pat. No. 4,833,696, to Iwanami, discloses a laminate with improved flex crack resistance, drawability and modality, and excellent gas impermeability. The laminate comprises a first layer of an EVOH copolymer and a thermoplastic polyester, and a second layer of a composition consisting of a group including nylon.
U.S. Pat. No. 4,855,178, to Langley, discloses a chemical barrier fabric in which a fabric material is laminated to a multilayer film of a layer of EVOH sandwiched between two layers of nylon. A heat-sealable polyethylene layer is disposed on the outside of the sheet material.
U.S. Pat. No. 4,909,726, to Bekele, teaches a multilayer polymeric film for chub packaging of the following layer structure:heat seal/abuse layer/tie/nylon/EVOH/nylon/tieThe heat seal layer may be selected from the group consisting of ethylene alpha-olefin copolymer, LDPE, and ethylene ester copolymer, and may include an antiblocking agent. The abuse layer may be a very low density polyethylene or alternatively may be an ionomer.
U.S. Pat. No. 4,983,431, to Gibbons, teaches in FIG. 3 a five-layer polymeric film which is laminated to a substrate such as paperboard. The film has the following layer structure:
 ionomer/nylon/EVOH/nylon/ionomer
One ionomer layer is laminated to the substrate while the second ionomer layer is coated with a layer of LDPE.
U.S. Pat. No. 4,937,112, to Schirmer, teaches a multilayer blown polymeric film for use in chub packaging. The film has a first outer layer of a heat sealable polymeric resin, such as LLDPE, a first interim layer comprising a polymeric material of high molecular weight such as LDPE, HDPE or EVA, a second interim layer comprising a nylon, and a second outer layer comprising a self-weldable polymeric material.
U.S. Pat. No. 4,999,229, to Moritani, teaches a multilayered polymeric gas barrier film with an intermediate layer of a composition having 50 to 97 weight percent EVOH and 45 to 3 weight percent nylon. As disclosed in Example 1, a layer of nylon may be disposed in contact with the intermediate Layer.
U.S. Pat. No. 5,068,077, to Negi, teaches a multilayer polymeric film with a barrier layer of from 70 to 95 weight percent EVOH and from 5 to 30 weight percent nylon. The barrier layer may be sandwiched between two layers of nylon.
U.S. Pat. No. 5,194,306, to Blatz, teaches a polymeric blend of a major portion of an amorphous nylon and a minor portion of EVOH for use as a gas barrier in a multilayer polymeric film. In an alternative embodiment at column 6, lines 8 to 11, the invention may comprise a two layer structure with one layer of substantially EVOH and one layer of substantially amorphous nylon. At column 8, example 20 discloses a three layer structure comprising a core layer of EVOH sandwiched between two nylon layers.